Low metallicity and ultra-luminous X-ray sources in the Cartwheel galaxy

Understand

Low-metallicity (Z <~ 0.05 Zsun) massive (>~40 Msun) stars might end their life by directly collapsing into massive black holes (BHs, 30 <~ m_BH/Msun <~ 80).

• More than ~10^5 massive BHs might have been generated via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ~10^7 yr.
• We show that such BHs might power most of the ultra-luminous X-ray sources (ULXs) observed in the Cartwheel.
• We also consider a sample of ULX-rich galaxies and we find a possible anti-correlation between the number of ULXs per galaxy and the metallicity in these galaxies.